Microwave polarization changer



1956 A.. J. SIMMONS 2,772,400

MICROWAVE POLARIZATION CHANGER Filed Jan. 8, i954 3 Sheets-Shes; 1

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D INVENTOR A LA'N J. SIMMONS BY i /WI? ATTOR NEYJ Nov. 27, 1956 A. J.SIMMONS ,77

MICROWAVE POLARIZATION CHANGER Filed Jan. 8, 1954 3 Sheets-Sheen 2 QT MMI W MH UTE ALAN J. SIMMONS flw/lgfig ATTORNEY5 NOV? 1956 A. J. SIMMONSMICROWAVE POLARIZATION CHANGER 3 Sheets-Sheerv 3 Filed Jan. 8, 1954United States Patent 015cc 2,772,400 Patented Nov. 27, 1956 MICROWAVEPOLARIZATION CHANGER Alan J. Simmons, Hyattsville, Md., assignor to theUnited States of America as represented by the Secretary of the NavyApplication January 8, 1954, Serial No. 403,081

6 Claims. (Cl. 333-21) (Granted under Title 35, U. S. Code (1952), sec.266) This invention relates in general to phase shifting devices andmore particularly to rectangular waveguide devices for changing thephase between orthogonally polarized waves in a linear polarization toproduce a circular polarization.

It is commonly known that circular polarization may be set up fromlinear polarization when two orthogonally polarized waves of equalamplitude in a transverse mode are shifted in relative phase one quarterwavelength. In the prior art several devices have been developed toproduce the necessary phase shift between the orthogonally polarizedwaves. The phase velocity of either one wave or the other has beenretarded to produce the one quarter wavelength phase delay. One familiardevice loads the waveguide uniformly by the insertion of a longitudinalmetal fin which alters the phase velocity of the wave parallel to thefin. Another device loads the waveguide periodically by the insertion ofsmall screws at intervals within the guide to alter the phase velocityof only one wave. Still another device in the prior art has a dielectricplate longitudinally positioned within the waveguide to retard the phasevelocity in both polarizations. In this device the phase delay isdependent on the dielectric material used and on a ratio of the amountof dielectric in the path of each wave. It is readily seen by thosefamiliar in the art that said dielectric plate will present a seriousmatching problem unless properly designed. Unfortunately a variation ofthe plate configuration will usually introduce a difiicult phase delaydetermination problem which may tend to offset any matching advantageobtained.

While waveguide transmission lines are relatively broadband polarizationchanging devices for use with these lines, with the exception of theabove described dielectric plate device, are generally narrowband.Whereas a broadband polarization device of simple design is particularlydesirable in many applications:

It is an object of this invention to provide a unique waveguide phaseshifting device with a wide band pass characteristic for translatinglinear polarization to circular polarization.

It is also an object of this invention to provide a means for producinga phase shift between orthogonally polarized waves of a linearpolarization whereby one Wave is advanced and the other wave is delayedto produce a relative phase shift.

It is an additional object of this invention to provide a broadbandwaveguide phase shifting device which may be simply and sturdilyconstructed to produce any amount of phase shift desired.

It is still another object of this invention to provide a means forproducing a phase shift to be used in the translation of linearpolarization to circular polarization within a rectangular waveguide inwhich matching means are incorporated as an integral part thereof.

Other objects of the invention will become apparent from a betterunderstanding of the invention for which reference is had to theaccompanying detailed description and drawings of the invention.

In the drawings:

Figure 1 is a side view of one embodiment of a phase shifting deviceaccording to this invention;

Figure 2 is an end view of the embodiment of Figure 1;

Figure 3 is an elevational view, cut away and in perspective to showdetail and assembly of the embodiment as shown in Figure 1;

Figures 4a, 4b, and 4c are a group of graphs showing the phase shift(no) between irises for various dimensional parameters in the describedembodiment.

Figures 5a, 5b, and 5c are a group of graphs showing the voltagestanding wave ratio (V S W R) for various dimensional parameters in thedescribed embodiment.

In the examination of the wave energies within the waveguide device ofthis invention only the transverse electric (TE) field, in which theelectric field lies across the waveguide and no E lines point in alongitudinal direction within the guide shall be considered. Therelative phase difference between modes discussed herein shall refer tothe phase difference between the orthogonally polarized TEn,1 and theTE1,0 modes. It is understood that the discussion which follows mightalso be applicable to a transverse magnetic (TM) field analysis.

Briefly, the invention comprises a periodic loading of a rectangularwaveguide by means of a series of windows, hereafter called irises,transversely disposed within the waveguide section. Each pair of irisescapacitively affects (delays) one mode and inductively affects(advances) the other mode to produce its proportionate phase shift, thetotal phase shift being the sum of the individual phase shifts withinthe waveguide section. With a change in frequency thiscapacitive-inductive efiect self-compensates to give the invention acharacteristic constant phase shift over a wide band of frequencies. Inthe device of this invention the amount of phase shift is dependent onthe dimensions of the waveguide, the size and separation of theincorporated irises, the number of irises and the free space wavelengthof a chosen center frequency.

Referring now to the drawings in detail:

in Figure 1 a rectangular waveguide section is shown according to thisinvention in which thin metal fins have been transversely placed asobstacles at determined intervals within the section to form a series ofirises. In the embodiment as shown there are 10 metal fins forming 5irises within the section. All the transverse fins are shown equallyspaced along the tube and each is of similar thickness and height withthe exception of the 4 end fins which are reduced in height to provide aproper matching impedance for the end section, the fins being so spacedand dimensioned for reasons which shall appear more fully hereafter.

The invention may incorporate any number of irises, dependent upon theamount of phase shift desired. In a particular waveguide section thespacing between irises may be reduced provided the non-progating fieldproduced by one iris do not interact with the corresponding fields ofthe preceding or following iris. It is of great importance in the basicanalysis of the invention that the susceptance of each iris may bedetermined independent of the next and it is obvious that anyinteraction between the adjacent irises would complicate such asimplified analysis. It is understood that the invention itself is notto be so operatively limited in the spacing of the irises. Attention iscalled merely to show the point at which a simplified analysis of thedevice may become more difficult.

In Figure l the fins are shown penetrating the walls of the wave guidein accordance with a simplified construction procedure wlrereny the finsare inserted in narrow slots and then soldered in place before the usualsilvering From a complex matrix formulation it may be shown of the unit.It is understood that it is not essential to the invention that the finspenetrate the walls of the wavel cos [COS Bl Bl] (4) guide and that thisinnovation is shown merely to display T above formulation has eonsldeledbotb B a11d S a preferred and simplified means by which s invention. 5as lndependent variables. In the broadbandmg analysis, may beconstructed however, it is essential that the dependence of these two Asshown in the drawing of Figure 1, the distance be- Variables onfrequency 011 free Space Wavelength tween the irises is represented by land the distance across be considered Where M is the Wavelength tbeguide the inside of the waveguide section is represented by b. for Onemode, M is the Wavelength in the guide for the Likewise, the distancebetween opposite fins of each of (Fiber mode and b the heigb'E 0f guideit Can be the center irises is represented by d and the correspondingSbeWIl that distance in the case of the larger end irises by e. The 21rl21rl b separate end view of the waveguide, shown in Figure 2, BC "1':T'X C (5) illustrates the transverse positioning of the fins within andthe section. The width of the waveguide section is repre- 2 l 2 l bsented by a as shown in this end view. B Z= (6) Figure 3 shows a portionof the embodiment of Figure 1 in a prgjected end iew. This utaway iewmore Where a iS th Width of the guide it can be SllOWll that clearlyportrays the transverse metal fins in their correb 1 b i 2a 2 sponding'relation to one another. 1 (7) Figures 4a4c and Sa-Sc will aid in theunder- 2 standing of the operation of this invention which is deandscribed hereafter in more detail. b 1 2a b 2 t:In alrlialyzingfthf'ioperation of thlis ingentilon thehetifect T ')(E) o eac pair o a 'acentirises wit in t e piase s iting device should be ccinsideredindividually. In this analysis Substituting Equanon 4 m Equatlon 2 theeifect of each intermediate iris is considered twice, A0c=cos [cosficl-Sc sin ficl]l3cl (9) once with "each of its adjacent irises. Forthe purpose when;v substituting Equation 7 in Equation of simpleanalysis an infinitesimal fin thickness is assumed and the basic theoryof inductive and capacitive irises of Z 2a 2 Zero thickness (asdescribed in Waveguide Handbook- 561:7- E (10) Marcuvitz M. I. T. Rad.Lab. Series volume 10, pages 218430) is applied to determine the phaseadvance and 1 phase delay contributing to the proportionate phase shiftB=C0S [cos flLl-SL sin ml] /3Ll (11) for each pair of irises. A shortsummary of the theory where substituting Equation 8 in Equation 6:

involved in this unique application of said basic theory I of admittanceand susceptance is outlined below. For a l=L\/ 1 (12) more comprehensiveunderstanding of the properties of b a thin irises reference is had tothe Marcuvitz text.

Similarly, substituting Equation 4 in Equation 3 Thus it can be seenthat an expression for the capaci- Typically, a loaded waveguide withadmittance Yo, tive phase delay A00 for a pair of irises may be obtainedused for calculating reflecting properties, and propagation in terms ofwaveguide height, width, admittance and susconstant [2, used forcalculating phase shift, may be receptance from a simple substitution ofEquation 10 in placed by an equivalent transmission line of new char-Equation 9. acteristic admittance Y0 and new propagation constantSimilarly it can be seen that an expression for the [3. For the purposeof this description 2S represents inductive. phase advance ABL. may beobtained in the same either the positive or negative susceptance of eachiris. terms from a substitution of Equation 1'2 in Equation 11. In thediscussion 28 also represents the susceptance of The susceptance ofstandard waveguide elements, such the phase shift section (betweenirises) but it should be as capacitive and inductive irises, has beenmeasured and understood that this is considered to be the summationrecorded in. such readily available sources as the reference of one halfthe susceptance of the preceding and of the text (pages 220223). It isconvenient, therefore, to following iris. Accordingly, it follows thatthe end irises consult such graphs to determine the values of thecapacishould be considered to have a susceptance S. tive susceptance Soin Equation 9 and the inductive sus- As previously referred to, acompensating capacitiveceptance SL in Equation 11 and to observe thedependence inductive efiect is inherent in the operation of this invenofSe and St. on the iris aperture and on the waveguide tion. Each of thethin irises acts as a shunt capacitance height, width and free spacewavelength. for one mode to introduce a phase delay and as a shunt Bysubstituting the expressions thus obtained for A inductance for theother mode to introduce a phase adand A01. in Equation 1', the totalrelative phase shift for vance. The proportionate phase shift, A t, foreach pair each pair becomes at t a s srsteer01-weei of irises is thesummation of the capacitive delay A00 5 The total phase. shift for thedevice is the sum of the and the inductive advance A01. as individual;phase shifts for each adjacent pairs of irises. v For identical;intermediate. iris, dimensions as shown in A(I IMCl+|A0Ll (1) theillustrated embodiment, the calculation of the phase Considering theequivalent transmission line, the 0&- shift of one pair will hold foreach of the others except Paeltive delay y be represented y the pairs,including the larger end irises. Thus it will be A0C:Bc,l 8cl (2)v seenvthat using only two different sized fins to form the irises: makesdetermination of the phase shift much easier.

In constructing the phase shifting device of this inven- =I -B (3): tionit is convenient, knowing the propagation constant wherel is the. actualseparation between irises. B for the medium, to first select a value forfil in the and the inductive advance by region of rr/Z and then, usingthe formulae disclosed herein; to determine the total number of phasesections between irises necessary .to give a 90 phase shift betweenorthogonal modes for the total line.

Using this convenient procedure for determining the phase shift thedistance between irises over the height of the guide, Z/b, will bearbitrary until the free space wave length of the chosen centerfrequency 7m and either I or b have been selected.

Considering bandwidth only, the proper size iris may be determined byuse of calculated graphs similar to those shown by Figures 4a-4c. Onthese graphs a bandwidth ratio for a 15% variation in phase shift isindicated. (Bandwidth ratio is the ratio of maximum to minimum value ofZa/Ao). In Figure 4a, Ad has been plotted for values of the parameterd/b for the center irises. Obviously a similar plotting might be madefor values of the parameter e/ b for the end irises. In Fig ure 4a it isseen that as the aperture of the iris is made larger the bandwidthincreases. In Figure 4b, no has been plotted for values of the parameterl/b. It is seen that as the ratio 1/ b is increased, the bandwidthgradually decreases. Figure 4c shows that a variation of the ratio b/afrom the value 1.0 decreases the bandwidth and also shifts the band ofoperation.

A further consideration in the design of this device is the maximumvoltage standing wave ratio (V S W R). Maximum V S W R refers to thevalue of (Yd/Yo) or (Yo/Y's) (whichever is larger) for an equivalenttransmission line having an electrical length which is an odd multipleof 1r/2. Y0'/Y0 is the ratio of equivalent line admittance to actualline admittance of one phase section and is essential to the calculation(using standard transmission line formulae) of the input admittanceYin/Y0 of the total waveguide section.

It may be shown:

In Figures 561-50, the maximum values of V S W R for the dimensionalvariation in Figures 4a4c are shown. A solid line shows the maximumpossible V S W R for the capacitively loaded mode and a dotted lineshows the maximum V S W R for the inductively loaded mode. Specifically,in Figure So it is seen that the V S W R is lower for a larger ratiod/b. While d/b should be chosen as large as feasible, it should be notedthat as d/b is increased the number of sections necessary for a givenamount of phase shift-also increases and the cumulative eliect ofwaveguide machining tolerances may tend to cancel the more apparentadvantages of a larger d/b.

In a typical instance of the disclosed embodiment of this invention, inwhich a 90il0% phase shift having a bandwidth as large as possible andcentered at 8300 me. u)=3.6 cm.) is desired, the following values aredetermined. Selecting l/b=0.40 for maximum bandwidth, with a minimumphase shift for the total four phase sections of 80 or for each phasesection a minimum phase shift of 20 and with both a and b=2.96 cm., thecenter of the band will be at 2a/)\o=1.65 and the iris spacing 1 will be1.19 cm. Choosing d/b=0.7 the center iris aperture d will be 2.07 cm.and the end iris aperture 6 will be 2.30 cm.

This invention as shown and described provides phase shifting devicehaving a wide bandpass characteristic which may be simply and sturdilyconstructed :to produce a readily determined relative phase shiftbetween orthogonally polarized waves in a waveguide transmission system.it is understood that this invention is adaptable to any square or nearsquare waveguide section and :that it is not to be restricted to thedisclosed square waveguide structure. Further, it is understood thatwhile all the fins are shown equally spaced and the intermediate onesare shown of equal height :these dimensions, being illustrative of onlyone embodiment, are not to be considered defin- 6 ing or limitingfeatures of the invention and that this invention is to be limited bythe scope of the appended claims alone.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the pay ment of any royalties thereon or therefor.

What is claimed is:

l. A waveguide polarization converter for shifiting the relative phaseof the orthogonal transverse modes of the wave to be converted whichcomprises: a waveguide sec tion, a plurality of conducting finstransversely mounted and longitudinally spaced at intervals therein,said fins being dimensioned and relatively spaced to delay one of saidmodes and to advance its orthogonal mode, the number of fins in saidplurality being suificient to provide a total relative phase shiftbetween said orthogonal modes of approximately degrees, and means forintroducing to the section an incident wave having orthogonal transverse modes of equal magnitude.

2. A waveguide polarization converter for shifting the relative phase ofthe orthogonal transverse modes of the wave to be converted whichcomprises: a substantially square waveguide section, a plurality of thinconducting fins transversely mounted and longitudinally spaced atregular intervals therein, said fins being dimensioned and relativelyspaced to delay one of said modes, and to advance its orthogonal mode,the number of fins in said plurality being sufficient to provide a totalrelative phase shift between said orthogonal modes of approximately 90degrees, and means for introducing to the section an incident wavehaving orthogonal transverse modes of equal magnitude.

3. A waveguide polarization converter for shifting the relative phase ofthe orthogonal transverse modes of the wave to be converted whichcomprises: a waveguide section, a plurality of pairs of thin conductingfins longitudinally spaced within said waveguide section, each of saidpairs being transversely disposed to form an aperture therein, said finsbeing dimensioned and relatively spaced to delay one of said modes andto advance its orthogonal mode, the number of fins in said pluralitybeing sufficient to provide a total relative phase shift between saidorthogonal modes of approximately 90 degrees, and means for introducingto the section an incident wave having orthogonal transverse modes ofequal magnitude.

4. A waveguide polarization converter for shifting the relative phase ofthe orthogonal transverse modes of the wave to be converted whichcomprises: a waveguide section, a plurality of pairs of thin conductingfins longitudinally spaced within said waveguide section, each of saidpairs being transversely disposed to form an aperture therein, each ofthe terminal pairs of said plurality having a greater aperture than anintermediate pair of fins in said plurality, said fins being dimensionedand relatively spaced to delay one of said modes and to advance itsorthogonal mode, the number of fins in said plurality being sufficientto provide a total relative phase shift between said orthogonal modes ofapproximately 90 degrees, and means for introducing to the section anincident wave having orthogonal transverse modes of equal magnitude.

5. A waveguide polarization converter for shifting the relative phase ofthe orthogonal transverse modes of the wave to be converted whichcomprises: a waveguide section, five pairs of thin conducting finslongitudinally spaced at regular intervals within said waveguidesection, each of said pairs being transversely disposed to form anaperture therein, said fins being dimensioned and relatively spaced todelay one of said modes and to advance its orthogonal mode, the numberof. fins in said plurality being sufficient to provide a total relativephase shift between said orthogonal modes of approximately 90 degrees,and means for introducing to the section an incident wave havingorthogonal transverse modes of equal magnitude.

6. A waveguide polarization converter for shifting the relative phase ofthe orthogonal transverse modes of the wave to be converted whichcomprises: a square waveguide section, five pairs of thin conductingfins transversely disposed to form apertures and longitudinally spacedat regular intervals within said waveguide section, the interval betweenpairs of fins being approximately 0.4 the transverse width of thewaveguide section, said fins being dimensioned to delay one of saidmodes and to advance its orthogonal mode, the number of fins in saidplurality being sufficient to provide a total relative phase shiftbetween said orthogonal modes of approximately 90 degrees, and means forintroducing to the section an incident wave having orthogonal transversemodes of equal magnitude.

References Cited in the file of this patent Publication I, Montgomery:Microwave Transmission Circuits, vol. 9, M. I. T. Radiation Lab. Series,published 1948 McGraw-Hill, pp. 369474. (Copy in Patent Office Library.)

Publication II, King, et al.: Transmission Lines, Antennas and WaveGuides, published 1949 McGraw-Hill, pg. 275.

